1. Field of the Invention
This invention relates to the art of processing endless webs of flexible materials such as typically polymer films or fibrous materials including nonwovens by continuously winding the moving web onto a sequence of winding cores to produce a sequence of web coils to facilitate handling, storing and further processing of the web material.
2. Description of the Prior Art
Methods and apparatus means for this purpose are well known in the art as disclosed e.g. in U.S. Pat. No. 4,191,341 (EP 17,277) or the references discussed therein and are frequently termed "winders", "web winding machines" or--if used for this specific purpose--"film winders".
A common feature or aim of the more advanced prior art winding methods is that relatively fast-moving webs (e.g. at web speeds of 30 to 300 meters/minute), such as polymer webs emanating from sheet extruders, blow tube extruders or web-processing lines must be wound up continuously, i.e. without interrupting the web stream, to produce an "endless" sequence of coils or web rolls. Generally, empty cores, typically in the form of cardboard tubes, are supplied from a magazine to a start-up winding position or "first station" and made to rotate therein while the web that is still being wound onto the preceding coil is in the main winding position or "second station". Now, the web will be cut transversely to terminate the preceding coil; the "trailing end" of the proceding web section will be on the top surface of that coil. The "leading end" of the subsequent web section is made to be picked up by the empty core in the start-up winding position or station (e.g. by an adhesive or electrostatic charge) while the preceding coil is discharged from the "actual" or main winding position. Then, the "start-up" coil produced in the start-up winding position is transferred without interrupting the winding operation into the main winding position and remains there until that coil has reached its predetermined volume and is terminated by again transversely cutting the web. This sequence from start-up to coil discharge is termed "winding cycle".
Rotation of the cores or of the web coils formed thereon can be achieved by a frictional contact between the coil surface and a driven winding drum in contact with the coil surface (winders of the circumferential or friction type) and/or by a separate drive that actuates the coil, e.g. by rotating its core (termed center winding).
In the first instance, a sufficient contact pressure must exist between the coil and the winding drum and such pressure ("line pressure") should be controllable since different pressures may be needed for different materials and/or different stages of coil completion.
In the second instance no such pressure is employed but a corresponding tractive power of the drive for the coil is used for winding.
Whether a given film web is more expediently processed using a friction winder or using a center winder usually depends on the material properties or the surface properties of the web material, e.g. polymer, and combination machines ("universal" or "multimode" winders) have been disclosed in the above cited art. Such machines are capable of operating either in the friction winding mode and/or in the central winding mode and provide for improved processing or improved economy.
A disadvantage of the prior art universal winders is that the so-called coil finish, that is to say the quality of the finished coil (typically having diameters of 100 to 1000 mm and widths of 5 to 3000 mm) may differ substantially depending on the polymer web material processed and on the operating mode of the winder by friction or center drive.
In order to control the linear pressure at the nip between the coil and the winding drum in the range of from a "zero pressure" and a predetermined positive contact pressure, the multimode winder disclosed in European Patent 17,277 is provided with a force sensor that permits measurement of the pressure exerted by the coil onto the winding drum; then, the contact pressure values thus determined are used to operate a compensator, e.g. a hydraulic cylinder, so as to keep the pressure between the winding drum and the wound coil at a desired predetermined or programme-controlled value, independently of the increasing weight of the supported coil pressing against the winding drum. The coil is supported "dynamically", i.e. held by a pair of arms in an angular position rather than "statically", i.e. supported in vertical direction.
Now, in order to control and maintain those winding conditions as are needed for a satisfactory coil finish with a given web material and notably for maintaining optimum conditions even though the diameter and the weight of the coil will increase during winding, winding operations required that the linear pressure to be maintained between the coil and the winding drum (in the nip), on the one hand, and the tensile force exerted in the web while moving onto the coil, on the other hand, be determined experimentally for each type of web material and each web thickness, and the values thus obtained must be used for computer-controlled operation of the winder.
In other words, the parameters of the winding operation are determined by a given programme that sets and regulates winding conditions for any given web. Thus, true control in the sense of measuring the actual critical parameters of operation and adjusting them as required for optimum coil finish is not possible with such systems.